The present invention relates to a purification method of cyclohexanone-oxime in solution.
Cyclohexanone-oxime is a chemical product which is used as intermediate for the production of caprolactam, whose preparation on an industrial scale can be effected by means of various processes known in the art (Ullmann""s Encyclopedia, 1986, Vol.A5, pages 31-50).
An additional process, called ammoximation, is described in U.S. Pat. No. 4,745,221 and comprises the preparation of cyclohexanone-oxime by reacting cyclohexanone with ammonia and hydrogen peroxide in the presence of a catalyst having a zeolitic structure and the isomorphic substitution of a number of silicon atoms with titanium atoms (titanium silicalite or TSl).
The cyclohexanone-oxime produced by means of this method or by means of any process, however, contains non-vapourizable residues in quantities ranging from 0.01 to 0.5% by weight. These values are obtained by evaporating the oxime at 120xc2x0 C. and 10 mm Hg of pressure.
The presence of these residues does not influence the subsequent transformation into caprolactam by means of Beckmann re-arrangement according to the classical industrial method in which oleum is used (Ullmann""s Encyclopedia, 1986, Vol.A5, pages 31-51). The residues, on the other hand, have negative effects on the catalytic Beckmann re-arrangement carried out in vapour phase (U.S. Pat. No. 4,141,896; EP 550965).
With this latter method, the cyclohexanone-oxime which is fed in vapour phase to the reactor, must contain a negligible percentage of non-vapourizable residues.
In order to simplify the catalytic Beckmann re-arrangement, in fact, it is very important to minimize the formation of non-vapourizable solid deposits on the walls of the equipment in which the cyclohexanone-oxime is contained in vapour phase.
These deposits not only foul the apparatus walls but can also excessively foul the catalyst, thus increasing the normal number of regeneration cycles of the catalyst.
Furthermore inefficiency in the running of the catalytic re-arrangement reactor may occur making it necessary to keep the cyclohexanone-oxime in liquid state at a high temperature, in the upstream equipment, or even in vapour phase for longer times than those normally required for vapourization alone and immediate feeding to the catalytic re-arrangement reactor.
A purification method of cyclohexanone-oxime in solution has now been found which allows the content of non-vapourizable residue to be significantly reduced, consequently obtaining a product which is particularly suitable for being fed to a catalytic re-arrangement process in vapour phase.
The object of the present invention therefore relates to a purification method of cyclohexanone-oxime in a solution of solvents immiscible with water which consists in washing said solutions with water or with an aqueous solution of a base having a pK less than 5 or in passing the solutions of cyclohexanone-oxime through a weakly alkaline ion exchange resin and, optionally, also through a weakly acid ion exchange resin.
The method is applied to solutions of cyclohexanone-oxime dissolved in a solvent immiscible with water in which, however, the cyclohexanone-oxime has a sufficient solubility and this solubility is greater than that of cyclohexanone-oxime in water.
Solvents with these characteristics can be selected from the group consisting of aromatic hydrocarbons, cyclic or linear aliphatic hydrocarbons, chlorinated hydrocarbons, oxygenated hydrocarbons, such as for example toluene, benzene, cyclohexane, methylcyclopentane, hexane, trieline or isopropyl ether.
Toluene solutions are preferably used.
Solutions of cyclohexanone-oxime can be conveniently treated in a solvent having a content of cyclohexanone-oxime varying from 30 to 60and by weight.
The washing can be effected with water or with solutions of a base having a pK less than 5 or the solutions of cyclohexanone-oxime can be passed through a weakly alkaline ion exchange resin and, optionally, also through a weakly acid ion exchange resin, obtaining an analogous reduction of non-vapourizable residues.
Bases which can be used for the purposes of the present invention are, for example, selected from NaOH, KOH, Na2CO3, NH3 at concentrations ranging from 0.01 N to 1 N and more conveniently from 0.05 to 0.1 N.
The volume of aqueous solution of the base used for the washing depends on the concentration of cyclohexanone-oxime present in the toluene solution and can range within a ratio of 10:1 to 1:1 parts by weight of cyclohexanone-oxime/parts by volume of solution of the base and more preferably in a ratio of 3:1 to 1:1.
The washing of the toluene solution of cyclohexanone-oxime can be effected in continuous in an appropriate column of the type used for liquid-liquid extractions or using a mixer and subsequent decanter or a series of mixers and decanters.
Solutions of cyclohexanone-oxime in toluene are, for example, obtained from the extraction section of an ammoximation process where the cyclohexanone-oxime is recovered from the hydro-alcoholic reaction mixture, after distillation of the solvent, by means of liquid-liquid extraction.
In this case, it is convenient to effect the washing of the toluene solution of cyclohexanone-oxime with an aqueous solution of NaOH having a concentration ranging from 0.05 to 0.1 N at a temperature ranging from 70 to 80xc2x0 C.
After the washing, the aqueous solution of NaOH is recycled to the extraction section of the ammoximation process to recover the dissolved cyclohexanone-oxime, whereas the toluene solution of cyclohexanone-oxime is sent to the toluene stripping column from whose bottom cyclohexanone-oxime is obtained, to be used for the catalytic Beckmann re-arrangement to caprolactam.
In order to verify the washing effects of the solution of cyclohexanone-oxime, a laboratory test is used, adopting extreme thermal conditions to which the cyclohexanone-oxime can be subjected.
The cyclohexanone-oxime is heated for 5 hours to 200xc2x0 C. and subsequently vapourized at 120xc2x0 C. and a reduced pressure of about 10 mm Hg. After evaporation of the whole oxime, the non-vapourizable residue is quantified.
The residue is expressed as a % with respect to the quantity by weight of the oxime used for the test.